High efficiency ionizer assembly



June 27, 1967 JAMES E. WEBB 3,328,624

ADMINISTRATOR OF THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATION HIGH EFFICIENCY IONIZER ASSEMBLY 2 Sheet-Sheet 1' Filed June 4, 1964 E E m n 2 y 0 Am m mm 0 I E2; m I z w N o 8 Y W up w I 4 2 w 2 2 6 2 w 7/ PO a v o o o o o 0 0 m 0 O O I 0 O O O O O O O W 0 o o o 8 2 Q o O O O 0 2V r0 0 O 0 o o O 0 O 1 BY 9 m E 6 M '4 @441 ATTORNEYS June 27, 1967 JAMES E. WEBB 3,328,624

ADMINISTRATOR OF THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATlON HIGH EFFICIENCY IONIZER ASSEMBLY Filed June 4, l964 2 Sheets-Sheet 2 HAY EN .E'. GQLLAGHEQ INVENTOR.

ATTOQNEVS United States Patent 3,328,624 HIGH EFFICIENCY IONIZER ASSEMBLY James E. Webb, Administrator of the National Aeronautics and Space Administration, with respect to an invention of Hayden E. Gallagher, Malibu, Calif.

Filed June 4, 1964, Ser. No. 372,730 4 Claims. (Cl. 313-230) This invention relates generally to electric thrust engines of the contact ionization type, commonly termed ion engines, wherein an ionizable fuel is ionized prior to its acceleration and expulsion for thrust generation to propell a space vehicle. More particularly this invention relates to an improved low mass ionizing device for use in such an engine which results in increased engine efiiciency and makes it feasible to use such engines in applications wherein intermittent operation is required.

An ion engine includes an ion source called an ionizer, a series of electrodes which accelerate and focus the ions generated by the ionizer, and a neutralizing device for neutralizing the beam of ions developed by the engine. The ionizer is composed of a porous tungsten plug or strip which is heated to a temperature in the range of 1100" C. to 1300 C. for operation. A manifold is associated with the ionizer for supplying an ionizable fuel such as cesium to the ionizer. Each atom of the cesium fuel loses an electron and becomes a positive ion as it passes through the ionizer. I g

A key parameter employed in evaluating the efiiciency of electric thrust devices is the input power to thrust ratio of the engine, or in other words, how sucessfully the engine performs depends upon obtaining a maximum amount of thrust with a minimum electrical power input. A substantial portion of the electrical power required to operate an ion engine is used to heat the ionized. Since the amount of power required to properly heat the ionizer is dependent upon the mass of the ionizer, the specific heat of the material the ionizer is composed of, and the temperature change necessary to bring the ionizer to the proper operating temperature, it is apparent that a substantial decrease in ionizer mass results in a substantial decrease in heater power required to heat the ionizer. The amount of heater power required to heat the ionizer becomes particularly important when the ion engine is to be used in an application where intermittent operation isrequired, such as for example, attitude control of a satellite. A typical operating sequence of an ion engine employed for attitude control of a satellite would be: ionizer heating time 30 seconds, thrust producing time 2 seconds, engine off-time 8 minutes, and then the sequence would be repeated. Essentially all of the heater power required by an ion engine operating in this manner is consumed during the ionizer warm up time. Therefore, a substantial reduction in heater power required results in a drastic decrease in the power requirement placed on the satellite power supply. Providing space vehicles with an adequate power supply is a serious problem so any method of decreasing the requirements placed on such a power supply is a significant development.

A major source of energy or power loss in an ion engine in continuous operation is the heat radiated from the ionizer assembly. A reduction in the overall mass and size of the ionizer assembly reduces the heat radiating surface area and thus reduces this power loss due to radiation. In addition, the smaller ionizer assembly permits the use of support structures having smaller crosssectional areas and hence the heat loss due to conduction through the supports is reduced.

The present invention provides several approaches to achieving a low mass ionizer assembly. A preferred approach is to provide an ionizer assembly which includes a porous tungsten ionizer with a manifold embedded therein for distributing cesium fuel to the tungsten ionizer. Embedding the manifold in the ionizer results in an ionizer having high mechanical strength with minimum mass. This mode of construction also allows the other components of the ionizer assembly such as the heating element and heat shields to be made smaller and placed closer to the ionizer. This results in an ionizer which requires less heating power and the outer surface area thereof is reduced such that energy loss due to heat radiation is significantly reduced. Two alternate embodiments of the invention are disclosed which achieve significant mass reduction without embedding the manifold in the ionizer. This is accomplished by employing improved manifold techniques and heater design.

It is therefore, a principal object of this invention to provide an improved and more efficient ion engine for use in a space vehicle.

Another object of this invention is to provide an ion izer assembly for an ion engine that makes it feasible to use such engines in applications requiring intermittent operation. g

A yet further object of the present invention is to provide an ionizer assembly that has been greatly reduced in mass.

Other object and advantages of the invention described herein will become more apparent when considering the following detailed description in conjunction with the accompanying drawings wherein:

FIG. 1 is a pictorial view of an ionizer assembly supported on a mounting ring.

FIG. 2 is a cross-sectional view of the ionizer assembly of FIG. 2 taken along lines 22 of FIG. 1.

FIG. 3 is an enlarged partially broken away plan view of the tubular manifold removed from the ionizer.

FIG. 4 is an enlarged view illustrating the shielding around the ionizer.

FIGS. 5 and 6 are cross-sectional views of ionizer assemblies in which the components thereof have been constructed and arranged in such manner that the over-all mass of the ionizer assembly has been considerably reduced.

Referring now to the drawings wherein FIG. 1 illustrates an ionizer assembly 10 supported upon a mountingring 12 by means of supports 14. An ionizable fuel such as cesium is supplied to the ionizer assembly through feed tubes or headers 16. The ionizer assembly consists of an ionizer 18 composed of porous tungsten, a tungsten manifold 20, a tungsten heating element 22 surrounded by electrical insulation 24 composed of alumina and a plurality of spaced reflective sheets 26 attached around the outside of said ionizer assembly. Manifold 20, as more clearly illustrated in FIG. 3, consists of a plurality of tungsten tubes having holes 28 formed therein and these tubes extend throughout the ionizer. The manifold tubes are fitted into apertures 17 formed in header 16. Cesium from a suitable source (not shown) is supplied to these tubes by header 16. Even distribution of the cesium to the ionizer is assured by varying the size of holes 28 in the manifold tubes, the hole size being increased as the dis tance of the hole from one of the headers increases. Supports 14 are composed of tantalum and they are attached at their upper ends to ionizer 18 and at their lower ends to mounting ring 12.

Referring now to FIG. 4 which illustrates the manner in which the heat shielding is attached to the ionizer. The heat shield consist of a series of reflective molybdenum sheets 26 which have been pierced at intervals to provide protrusions 30 which extend therefrom and maintain a desired spacing between sheets 26. The reflective sheets provide protection against heat loss due to radiation. The heat shielding is retained in place by attaching the innernost sheet to ionizer 18. The outermost sheets are held .n position by the protrusions 30 and tungsten strap mem- Jers 32 secured across the upper edges of sheets 26. Only one strap member 32 is illustrated; however, a desired number of these straps are spaced at proper intervals to secure the heat shielding. It should be noted that the spacing and size of the reflective sheets 26 in FIGS. 4 have been greatly exaggerated for purposes of illustration. In actual construction the heat shield would more nearly approach the appearance shown in FIGURES 2, 5, and 6.

While each of the components of the ionizer assembly have been described as being composed of a preferred specific material it should be understood that any suitable refractory metal such as rhenium, tantalum or molybdenum, or an electrical insulating material such as magnesia or beryllia, can be used in fabricating the above discussed components.

Referring now to FIGS. 5 and 6 of the drawing which illustrates two further embodiments of the invention which result in a low mass, high efficiency ionizer assembly. The mass reduction in these embodiments is not as great as the previously discussed embodiment. However, mass reduction is still sufficient to permit an ion engine employing such an ionizer to be used in applications where intermittent operation is required.

The ionizer assembly illustrated in FIG. 5 consists of a rectangular or strip type porous tungsten ionizer having an upper arcuate surface 36, a lower arcuate surface 38 and two vertical side surfaces 40 and 42. A thin sheet manifold 44 is attached to the two vertical side surfaces of the ionizer to form a trough shaped reservoir of cesium beneath the ionizer. The cesium is supplied to the manifold by header 46. The cesium is supplied at sufficient pressure so that it will pass from the lower arcuate surface of the ionizer through the ionizer to the upper arcuate surface which is the ion emitting surface. The ionizer assembly is mounted by means of support members 48 and 50 which are attached to the sides of the manifold. Heating element 52 is mounted in the bottom of the manifold and consists of tungsten conductor 54, a layer of alumina 56 which electrically insulates the conductor and a heater cover 68 composed of tantalum.

Another embodiment of the invention is illustrated in FIG. 6. This embodiment differs from that of FIG. 5 in that the manifold has curved or arcuate portions 60 and 62 formed in the side thereof in which the heater elements are mounted. Support members 64 and 66 include arcuate portions 68 and 70 which fit over the heating elements when the support members are attached to the manifold.

Mass reduction of the ionizer assemblies illustrated in FIGS. 5 and 6 is accomplished primarily by stamping the manifold from very thin sheets, 0.001 to 0.005 thick, of refractory metal and by judicious choice and placement of the heating elements. For example, heretofore ionizer assemblies have employed manifolds which were machined from billets and the typical wall thickness thereof was 0.040 inch. Previously used heating elements consisted of a tungsten coil sandwiched between insulating material and these heating elements had a cross-sectional area of about 0.2 inch to 0.2 inch. The circular heating elements employed in the present invention are about 0.040 inch in diameter. It is readily apparent that an ionizer assembly constructed in accordance with the principles of the present invention would result in a reduc- 4 tion in mass and overall dimensions and thus substantially reduce the heater power required for operation.

This completes the detailed description of the invention. While only preferred exemplary embodiments of the invention have been described herein it should be understood that there will be many changes and modifications which can be made thereto without departing fromthe spirit and scope of the invention as defined by the appended claims.

What is claimed is:

1. An ion emitting device of the contact ionization type that includes:

(a) a source of ionizable fuel,

('b) an ionizer having an ion emitting surface,

(c) a first means connected to said fuel source and said ionizer for receiving and distributing the ionizable fuel to said ionizer,

(d) a second means mounted adjacent the ionizer for heating said ionizer to a desired temperature,

(e) heat shielding means disposed around said ion emitting device and attached thereto so as to enclose said first and second means and said ionizer except for the ion emitting surface which is exposed,

(f) said heat shielding means comprising a plurality of spaced, reflective sheets of refractory metal, and

(g) each of said spaced sheets having protrusions extending therefrom to maintain spacing between said sheets.

2. An ion emitting device of the contact ionization type for use in an electric thrust engine, said device comprising:

(a) an ionizer in communication with a source of ioniza-ble fuel,

(b) a manifold composed of a plurality of perforated tubes embedded within said ionizer for receiving and distributing the fuel within said ionizer.

3. The ion emitting device recited in claim 2 wherein:

(a) said ionizer is porous tungsten, and

(b) said perforated tubes are tungsten.

4. An ion emitting device comprising:

(a) a porous ionizer having an upper and lower arcuate surface and two vertical side surfaces,

(b) a manifold composed of a plurality of perforated tubes embedded within said ionizer,

(c) a plurality of headers connected to said manifold for delivering fuel from an ionizable fuel source to said manifold where it is distributed within said ionizer,

(d) a layer of electrical insulating material abutting the lower arcuate surface of said ionizer,

(e) heater means imbedded in said layer of insulating material for heating said ionizer to a desired temperature,

(f) shielding means attached to said ionizer and covering the vertical side surfaces thereof and the insulating material whereby heat loss from the ion emitting device is substantially reduced.

References Cited UNITED STATES PATENTS 3,117,416 l/l964 Harries 3l363 X 3,210,926 10/1965 Forbes et a1. 3l3-63 X JAMES W. LAWRENCE, Primary Examiner. S. A. SCHNEEBERGER, Assistant Examiner. 

2. AN ION EMITTING DEVICE OF THE CONTACT IONIZATION TYPE FOR USE IN AN ELECTRIC THRUST ENGINE, SAID DEVICE COMPRISING: (A) AN IONIZER IN COMMUNICATION WITH A SOURCE OF IONIZABLE FUEL, (B) A MANIFOLD COMPOSED OF A PLURALITY OF PERFORATED TUBES EMBEDDED WITHIN SAID IONIZER FOR RECEIVING AND DISTRIBUTING THE FUEL WITHIN SAID IONIZER. 